1 Foundations of Nutrition Science

What nutrition science examines

Nutrition science examines how food and its components affect the body, health, and disease. It draws on knowledge of biology and metabolism and evidence from dietary assessment, laboratory research, clinical studies, and population studies.

Nutrition can support growth, tissue repair, and normal body functions. Health also depends on factors such as genetics, environment, access to food, physical activity, and medical conditions.

Nutrients and their roles

Nutrients are grouped by the amounts the body needs. Macronutrients—carbohydrates, fats, and proteins—are needed in relatively large amounts and provide energy and materials for body structures and processes. Water is also needed in large amounts, although it does not provide energy. Micronutrients—vitamins and minerals—are needed in smaller amounts and help regulate bodily processes; they do not provide calories.

Essential nutrients must be obtained from the diet because the body cannot make enough of them to meet its needs.

Carbohydrates, proteins, fats, and water

Carbohydrates include sugars, starches, and fiber, and provide about $\frac{4\ \text{kcal}}{\text{g}}$. The body can break down many carbohydrates into glucose, an important fuel for cells. Fiber is not fully digested; found in foods such as beans, vegetables, fruits, and whole grains, it supports bowel function and can influence blood cholesterol and glucose.

Proteins provide about $\frac{4\ \text{kcal}}{\text{g}}$ and are made of amino acids. They help build and repair tissues and form enzymes, some hormones, transport molecules, and antibodies. Sources include beans, lentils, soy foods, nuts, seeds, eggs, dairy foods, fish, poultry, and meat.

Fats provide about $\frac{9\ \text{kcal}}{\text{g}}$, making them the most energy-dense macronutrient. They supply essential fatty acids, form part of cell membranes, help absorb vitamins A, D, E, and K, and provide stored energy. Sources include oils, nuts, seeds, avocados, fish, dairy foods, and some meats. The type and overall pattern of fat intake matter to health.

Water supports chemical reactions, transports substances, helps regulate body temperature, and contributes to blood volume. It comes from drinks and foods, as well as from a small amount produced during metabolism.

Vitamins, minerals, and nutrient needs

Vitamins are organic compounds, while minerals are inorganic elements. Both have diverse functions. Iron helps carry oxygen in blood; calcium contributes to bones and teeth and supports muscle function; vitamin C supports connective-tissue formation and helps the body absorb iron from plant foods; and vitamin A supports vision and normal cell function.

Food sources vary. Leafy greens provide folate and vitamin K, citrus fruits provide vitamin C, dairy foods and fortified alternatives can provide calcium, and legumes provide iron and other minerals. Fortified foods can also contribute important nutrients.

Nutrient needs vary with age, sex, and life stage. Dietary Reference Intakes (DRIs) are reference values used to plan and assess nutrient intake, including values such as the Recommended Dietary Allowance and the Tolerable Upper Intake Level. Both too little and, for some nutrients, too much can be harmful. A varied eating pattern is generally a practical way to obtain a range of nutrients. Supplements may be useful in particular circumstances, but do not replace the full range of components found in foods.

How digestion makes nutrients available

Digestion mechanically and chemically breaks food into smaller components that can be absorbed. Chewing begins mechanical digestion, and enzymes in saliva begin breaking down some carbohydrates. In the stomach, acid and enzymes help digest proteins.

In the small intestine, pancreatic enzymes and enzymes on the intestinal surface continue breaking down carbohydrates, proteins, and fats. Bile helps disperse fats. Most nutrient absorption occurs through the small intestine into the blood or lymph. The large intestine absorbs water and is also home to microbes that ferment some fibers.

How the body uses and stores nutrients

Metabolism is the network of chemical reactions that maintains life. After absorption, nutrients may be used immediately, converted into other compounds, or stored for later use. Cells use glucose to make usable energy, and excess energy can be stored, including as body fat. Vitamins and minerals help many metabolic reactions proceed, but they do not themselves supply energy.

Energy balance over time

Energy in food is commonly expressed in kilocalories. For estimating energy, carbohydrate and protein provide about $\frac{4\ \text{kcal}}{\text{g}}$, fat provides about $\frac{9\ \text{kcal}}{\text{g}}$, and alcohol provides about $\frac{7\ \text{kcal}}{\text{g}}$. Alcohol provides energy but is not an essential nutrient.

Energy balance describes the relationship between energy consumed and energy expended over time. Expenditure includes energy used at rest, during physical activity, and in digesting and processing food. When intake and expenditure are broadly matched over time, body energy stores tend to remain relatively stable. A sustained surplus can increase energy stores, while a sustained deficit can reduce them.

Energy balance is not a simple, fixed calculation: appetite, activity, body composition, health, and other factors influence both intake and expenditure. It is one part of health, not a complete measure of it.

Estimating dietary intake

Dietary assessment estimates what a person or group eats and drinks. Common methods include a 24-hour dietary recall, a food record kept over several days, and a food-frequency questionnaire about usual intake.

Each method has limitations. People may forget foods, misjudge portion sizes, or change what they eat while recording it. Reported intake is therefore interpreted alongside other information, such as health history, physical measurements, clinical findings, and laboratory tests when appropriate. A single day of eating does not necessarily represent a person's usual diet.

Evaluating nutrition evidence

Different study designs answer different questions. Controlled trials can test the effects of a specific dietary change under defined conditions. Observational studies can examine dietary patterns and health across larger populations, but cannot by themselves prove that a food caused an outcome.

When weighing evidence, researchers consider study design, measurement quality, consistency of findings, and plausible biological mechanisms. This evidence informs dietary guidance, clinical care, food policy, and research into how dietary patterns relate to health.